Biological thiols are essential in maintaining the appropriate redox status of proteins, cells and organisms (Wood et al., Trends Biochem. Sci. 2003, 28, 32-40; Schultz et al., Eur. J. Biochem. 2000, 267, 4904-4911). Cysteine (Cys) is an essential amino acid that is involved in protein synthesis, detoxification, and metabolism. Elevated levels of Cys have been associated with neurotoxicity (Wang et al., J. Neurosci. 2001, 21, 3322-3331). Cys deficiency is involved in slowed growth, hair depigmentation, edema, lethargy, liver damage, muscle and fat loss, skin lesions, and weakness (Shahrokhian, Anal. Chem. 2001, 73, 5972-5978). Homocysteine (Hcy) has been implicated in various types of vascular and renal diseases. Elevated Hcy (e.g., >12 μM) in blood is a well-known risk factor for cardiovascular, Alzheimer's disease, neutral tube defects, complications during pregnancy, inflammatory bowel disease, and osteoporosis (Seshadri et al., N. Eng. J. Med. 2002, 346, 476-483; Refsum et al., Annu. Rev. Med. 1998, 49, 31-62). Therefore, the determination of Cys and Hcy in vivo is correlated to physiological functions in diagnosing disease. However, because Cys and Hcy levels are associated with different diseases despite their similar structures, a need exists for a method to discriminate between Cys and Hcy.
Significant effort has gone into the development of colorimetric, phosphorescent, and fluorescent probes for these thiol-containing amino acids to achieve high sensitivity, low cost and ease of detection. To date, most of the indicators or dosimeters are based on the strong nucleophilicity of the thiol group, and various mechanisms have been employed, including Michael addition, cleavage reactions, and others. Though these probes show high sensitivity toward thiol-containing compounds, the direct detection of Cys (or Hcy) is hampered due to interference from other thiols.
For example, Cys and Hcy are known to undergo cyclization with aldehydes to form thiazolidines (or thiazinanes). Because both the sulfhydryl and the amino groups contribute to the cyclization, aldehyde cyclization enables selectivity for Cys and Hcy over other common thiols such as glutathione (GSH). However, since the aminothiol moieties of Cys and Hcy have similar reactivities towards aldehydes in general, discrimination of them from each other is challenging using heterocycle formation (Li et al., Chem. Comm. 2009, 5904-5906; Lee et al., Chem. Commun. 2008, 6173-6175; Tanaka et al., Chem. Commun. 2004, 1762-1763; Duan et al., Tetrahedron Lett. 2008, 49, 6624-6627; Kim et al., Tetrahedron Lett. 2008, 49, 4879-4881; Zhang et al., Tetrahedron Lett. 2007, 48, 2329-2333; Zhang et al., Org. Lett. 2009, 11, 1257-1260; Lim et al., Chem. Commun. 2010, 46, 5707-5709).